Lead zirconate titanate, Pb(Zr.sub.x Ti.sub.1-x) O.sub.3 or PZT, ferroelectric thin film capacitors have been studied over the past few years as candidates for use in nonvolatile memories. The PZT capacitors are normally fabricated using either metal or oxide electrodes. The most commonly used metal electrode has been Pt, while the most commonly used oxide electrodes have been ruthenium oxide (RuO.sub.2) and lanthanum strontium cobalt oxide (La.sub.1-x Sr.sub.x CoO.sub.3). See, R. Ramesh et al., (1992), J. Electronic Materials, Vol. 23, p. 19, (1994); S. Bernstein et al., (1992), J.Mater.Res. 8,12; K. Bellur et al., (1992), Proceedings of the Eighth International Symposium on Applications of Ferroelectrics, IEEE publication No. 92CH3080-9, pp 448; D. Vijay et al., (1992), Proceedings of the Eighth International Symposium of Applications of Ferroelectrics, IEEE publication No. 92CH3080-9, 408; R. Dat et al., (1994), Appl.Phys.Lett. 64, 2673; O. Eom et al., (1993), Appl.Phys.Lett. 63, 2570; and O. Auciello et al., (1994), Appl.Phys.Lett. 64, 2873.
However, in both cases the PZT capacitors may have drawbacks which can hamper their use in nonvolatile memories. For example, while Pt/PZT/Pt capacitors have relatively low leakage current and relatively good dielectric breakdown properties, they may undergo severe polarization fatigue upon repeated switching. On the other hand, oxide/PZT/oxide heterostructures may have excellent resistance to polarization fatigue, but they usually have relatively high leakage currents as compared to Pt/PZT/Pt capacitors and may be more susceptible to dielectric breakdown.
Researchers have attempted to improve the fatigue characteristics of Pt/PZT/Pt capacitors using various approaches. These attempts included donor doping of the Pb(Zr.sub.x Ti.sub.1-x)O.sub.3 films as discussed in H. Watanabe et al., (1992), Proceedings of the Fourth International Symposium on Integrated Ferroelectrics pp 346, and J. Chen et al., (1992), Proceedings of the Eighth International Symposium on Applications of Ferroelectrics, IEEE No. 92CH3080-9, 111. These attempts also included changing the Zr/Ti ratio as discussed in H. Al-Shareef et al., (1994), Thin Solid Films, 252, 38 (1994), and G. Teowee et al., (1993), MRS Symp.Proc. 310, pp 423. Still other attempts included the growth of epitaxial PZT thin films as discussed in K. Bellur et al., (1992), Proceedings of the Eighth International Symposium on Applications of Ferroelectrics, IEEE publication No. 92CH3080-9, pp 448. Unfortunately, only marginal improvement in the polarization fatigue of Pt/PZT/Pt capacitors could be achieved.
There have also been attempts to reduce the leakage current of PZT films grown on oxide electrodes. In the case of RuO.sub.2, these attempts included annealing the bottom RuO.sub.2 electrode prior to PZT film deposition. H. Al-Shareef et al., Thin Solid Films, 256, 73 (1995). These methods have resulted in some improvement of the leakage characteristics of oxide/PZT/oxide capacitors.